Properly, they aren't really maps but rather 'bird's eye views' of particular places, although I don't think you can preclude the use of a map as a guide for the production of the bird's eye view. I've looked at (literally) thousands of pieces of letterhead and am always amazed at the excellence of the vignettes incorporated into the letterheads themselves, especially those that are views from above which predate manned flight! I've attached one example of a 'letterhead' (this one is from a bill head, but I'm certain they used the same vignette for both purposes) which I believe is a natural extension of the architectural rendering of buildings common in vignettes around the turn of the century. I hope this helps!
Honestly, not much has been done on letterheads. I too would love to hear of anything! There has however been some work on bird's eye maps. Isn't breaking new ground exciting?